Poly-ADP-ribose polymerase (PARP) existing in the nucleus is an enzyme which is activated by DNA degradation caused by inducible nitrogen oxide, oxygen radical and the like and thereby effects addition of the ADP ribose moiety of nicotinamide adenine dinucleotide (NAD) to nuclear protein and DNA. That is, a DNA damage activates PARP to cause addition of a large amount of ADP ribose to histone and PARP itself while consuming ATP. It is considered that intracellular NAD and ATP are reduced as the result to cause cell death due to intracellular energy depletion (Zhang, J. and Snyder, S. H., Science, 263, 687–689, 1995). Five genes coding for the PARP protein have so far been reported, and it is considered that the PARP activity is expressed through functioning of these genes. Among these genes, PARP-1 gene is the gene analyzed in most detail and is considered to be a gene having the most high contribution to the cell death accompanied by the activation of PARP protein (Smith, S., Trends Biochem. Sci., 26174–179, 2001).
It has been reported that NAD depletion by a DNA damage in cells of pancreatic islets of Langerhans does not occur in a PARP gene-deleted mutant mouse (Heller, B. et al., J. Biol. Chem., 270, 11176–11180, 1995). In addition, according to another reference, it has been revealed that the NAD depletion by a DNA damage does not occur also in cerebral cortex neuron in the aforementioned mutant mouse, and it has been reported that the cerebral infarction range accompanied by transient cerebral ischemia is significantly reduced in this mouse (Eliasson, M. J. et al., Nature Med., 3, 1089–1095, 1997). Thus, it is known that the cell death induced by the activation of PARP causes serious diseases in tissues of the living body. Also known as diseases in which the cell death induced by the activation of PARP is concerned include neuronal death at the time of cerebral ischemia, cell death of the heart after myocardial infarction reperfusion, autoimmune destruction of β-cells of pancreatic islets of Langerhans in type I diabetes mellitus, cell death after shock, inflammatory reaction by immunocyte death, and rheumatoid arthritis as an autoimmune disease which onsets by the generation of an abnormality in immune functions (Eliasson, M. J. et al., Nature Med., 3, 1089–1095, 1997; Zingarelli, B., Salzman, A. L. and Szabo, C., Circ. Res., 83, 85–94, 1998; Burkart, V. et al., Nature Med., 5, 314–319, 1999; Pieper, A. A. et al., Proc. Natl. Acad. Sci., 96, 3059–3064, 1997; Szabo, C., Cuzzocrea, S., Zingarelli, B., O' Connor, M. and Saltzman, A. L., J. Clin. Invest., 100, 723–735, 1997; Oliver F. J. et al., EMBO J., 18, 4446–4454, 1999; Szabo, C. et al., Proc. Natl. Acad. Sci., 95, 3867–72, 1998).
On the other hand, it is known that N-methyl-N′-nitro-N-nitrosoguanidine (MNNG) or H2O2 causes cell death by activating PARP (Halappanavar, S. S. et al., J. Biol. Chem., 274, 37097–37104, 1999; Watson, A. J., Askew, J. N. and Benson, R. S., Gastroenterology, 109, 472–482, 1995), and it is known that 3,4-dihydro-5-[4-(1-piperidinyl)butoxy]-1(2H)-isoquinolinone (DPQ), 3-aminobenzamide or nicotinamide inhibits cell death caused by PARP through the inhibition of the activation of PARP (Takahashi, K. et al., Brain Res., 829, 46–54, 1999; Watson, A. J., Askew, J. N. and Benson, R. S., Gastroenterology, 109, 472–482, 1995)).
However, the presence of a factor existing in the downstream of PARP has not been clarified yet, and it is not clear at present about the cell death-inducing mechanism of the PARP activation.
On the other hand, a human LTRPC2 (long transient receptor potential channel 2) gene was obtained in 1998 (Genomics, 54, 1, 124–131, 1998). Regarding a mouse LTRPC2 gene, there are reports stating that it was obtained (Japanese Journal of Pharmacology, Suppl. 1, 83, 2000, Molecular Cell, 9, 163–173, 2002), but its actual nucleotide sequence has not been reported. In addition, information on a rat LTRPC2 has not been reported. It has been revealed that the human LTRPC2 gene is activated by intracellular ADP ribose or NAD and functions as a calcium permeable non-selective cation channel (Sano, Y. et al., Science, 293, 1327–1330, 2001). Though it has been suggested that the controlling mechanism of LTRPC2 by ADP-ribose may have an important role in cellular functions (Nature, 411, 6837, 595–599, 2001), it is not clear about the functioning mechanism of LTRPC2 in cells and its subsequent influence upon the cells.
In addition, expression of the LTRPC2 gene in a cell prepared by cloning an immunocyte has been revealed, but its expression in actual human blood, particularly in immunocyte-containing lymphocyte, is not clear.
As has been described in the above, the fact itself that activation of PARP induces cell death is known, but the presence of a factor existing in the downstream of PARP is not clarified and its mechanism is unclear. Thus, great concern has been directed toward a method for screening a substance capable of inhibiting cell death induced by the activation of PARP, particularly a substance which is useful as a therapeutic agent and/or a preventive agent for a disease in which cell death induced by the activation of PARP is concerned (e.g., rheumatoid arthritis, neuronal death at the time of cerebral ischemia, cell death of the heart after myocardial infarction reperfusion, autoimmune destruction of β-cells of pancreatic islets of Langerhans, cell death after shock, or inflammatory reaction by immunocyte death).